Dark Colored Bruise Like Bulging Vein. Discomfort With Sharp Pain. What Is It?

occasionally , on my fingers, it appears as if a vein or something slips out of place in the joints. There is a bulge visible, darkly colored like a bruise , as the vessel shifts out of alignment, and it is accompanied by extreme discomfort and sometimes sharp pain. It returns to normal after about ten minutes to a half jour, on the shorter side if I m somewhere where I can ice it or run it under cold water. It normally happens between the second knuckles on my fingers, but right now had just happened on the top of my first knuckle on my index finger . What s happening?